Is this an infants swing? you can have holes drilled part way in at the angle of the legs so they would set in to the hole ( I would not recomend this one ).

If it is something that stays out doors then use a bracket that is shaped like an "L" with a screw into the leg of the swing and a screw into the deck. Or you can have a wooden box built like a photo frame that goes around the four legs not allowing it to move with rubber crutch tips on the legs of the swing also will keep it from sliding and moving around. Best of luck to you across the pond!